The present invention relates generally to vane-axial fans. In particular, the invention relates to a vane-axial fan whose flow characteristics may be changed by modifying its impeller and inlet shroud so that the operation of the fan will remain stable for reduced flow requirements.
Typical vane-axial cooling fans include a motor-driven impeller which propels a stream of air through a fan housing. Such fans may include an outlet guide vane assembly which is positioned downstream of the impeller and acts to both de-swirl and increase the static pressure of the air. These fans may further include a diffuser section which is located downstream of the outlet guide vane assembly and functions to decelerate and thereby further increase the static pressure of the air.
Generally, a cooling fan is designed to meet a specific flow and pressure rise requirement, which is called the design point, for a specific rotational speed, which is referred to as the design speed. At a particular design speed, the flow and pressure rise are related by the fan's characteristic curve, an example of which is shown in FIG. 2. The characteristic curve reveals that when a low pressure rise is required, the fan will deliver a high flow, but as the required pressure rise increases, the flow delivered will decrease. However, as the required pressure rise reaches a certain point, the fan will exhibit flow instability, which is commonly called stall. In many applications this instability may be potentially hazardous, and operation at the required design point is therefore not allowed. Instead, a different fan design will have to be used which is capable of producing the required pressure rise at a lower flow.